Method and system for improved reactant mixing and distribution

ABSTRACT

One aspect of the present invention relates to a mixing system for use in a chemical-process column. The mixing system includes a heavy-reactant mixing surface arranged perpendicular to a flow of reactant through the chemical-process column. The mixing system also includes an aperture formed in the heavy-reactant mixing surface. A pre-distributor is coupled to an underside of the mixing system and fluidly coupled to the aperture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/449,122, filed Apr. 17, 2012. U.S. patent application Ser. No.13/449,122 claims priority to U.S. Provisional Patent Application No.61/476,494, filed Apr. 18, 2011. U.S. patent application Ser. No.13/449,122 and U.S. Provisional Patent Application No. 61/476,494 areeach incorporated herein by reference.

BACKGROUND

Field of the Invention

The present application relates generally to hydrotreating processes andmore particularly, but not by way of limitation, to improved methods andsystems for reactant mixing and distribution in hydrotreating reactors.

History of the Related Art

Hydrotreating refers to a class of catalytic chemical processes forremoving impurities such as sulfur, benzene, or the like from petroleumproducts such as, for example, gasoline, kerosene, diesel fuel, and thelike. The purpose of hydrotreating is to reduce emission of pollutantsthat result from combustion of petroleum products utilized in automotivevehicles, aircraft, railroad locomotives, ships, gas or oil burningpower plants, residential or industrial furnaces, and other forms offuel combustion.

An example of a common hydrotreating reaction involves removal of sulfurfrom various petroleum products. Such a hydrotreating reaction isreferred to as “hydrodesulfurization.” Hydrodesulfurization is ofparticular importance because sulfur, even in low concentrations, maypoison metallic catalysts such as, for example, platinum and rhenium,that are used in refining processes to upgrade an octane rating ofpetroleum products. Furthermore, sulfur dioxide (SO₂) results fromcombustion of sulfur-contaminated petroleum products. SO₂ is awidely-recognized pollutant, which has well-known and wide-rangingdetrimental effects on the environment.

Another example of a common hydrotreating reaction involves removal ofbenzene from petroleum products. In recent years, the Federal governmenthas specified maximum acceptable quantities of benzene that may bepresent in petroleum products. Thus, refinery operators are required toeither capture or destroy benzene present in petroleum products. Captureof benzene represents a substantial capital investment for a refinery.Hydrotreating of benzene is a substantially cheaper alternative andinvolves reacting a benzene-contaminated petroleum product with hydrogenvapor (H₂) in the presence of a catalyst. As a result, benzene isdegraded into cyclohexane.

In an industrial hydrotreating unit, such as those found in a refinery,hydrotreating takes place in a reactor column at temperatures rangingfrom about 300° C. to about 400° C. and pressures ranging from about 30atmospheres to about 130 atmospheres. Typically, hydrotreating takesplace in the presence of a catalyst. Typically, the catalyst is in theform of generally spherical pellets that are packed into variousportions of the reactor column known as packed beds.

In operation, reactants descend in a concurrent-flow manner through thereactor column. In most cases, small gaps are present between adjacentcatalyst pellets thereby allowing passage of reactants therethrough. Thehydrotreating reaction occurs on a surface of the catalyst pellets. Inmany cases, during operation, reactants are consumed unevenly within thereactor column. Uneven consumption of reactants gives rise to aconcentration gradient. The concentration gradient, in many cases, isalso accompanied by a temperature gradient across a cross-sectional areaof the reactor column. For this reason, reactor columns often includevarious mixing and distribution devices.

U.S. Pat. No. 7,078,002, assigned to Shell Oil Company, describes amixing device for mixing fluids in a multiple-bed downflow reactor. Themixing device includes a substantially horizontal collection tray and aswirl chamber arranged below the substantially horizontal collectiontray for mixing liquid. The swirl chamber has an upper end part that isin direct fluid communication with an upper surface of the substantiallyhorizontal collection tray and an outlet opening at a lower end. Alength of the swirl chamber is at least 0.35 times its inner diameter.The mixing device further includes a substantially horizontaldistribution tray beneath the swirl chamber. The substantiallyhorizontal distribution tray includes a plurality of openings fordownward flow of liquid and gas.

U.S. Pat. No. 7,052,654, assigned to ExxonMobil Research and EngineeringCompany, describes a multi-phase mixing system for distributing vaporand liquid across a downflow reactor. The mixing system includes acollection tray for receiving vapor and liquid. The collection trayincludes a sub-region. The mixing system further includes a mixingchamber positioned below the collection tray and at least one outletopening for downward passage of vapor and liquid. The mixing systemfurther includes a conduit extending through the collection tray intothe mixing chamber. The conduit permits the flow of vapor and liquidfrom above the collection tray and into the mixing chamber. The mixingsystem further includes a vapor slipstream passageway extending throughthe upwardly projecting sub-region for permitting flow of a vaporslipstream from above the collection tray into the mixing chamber.

U.S. Pat. No. 7,045,103, also assigned to ExxonMobil Research andEngineering Company, describes a distributor system for distributingvapor and liquid across a downflow reactor. The distributor systemincludes a collection tray for receiving vapor and liquid and a mixingchamber positioned below the collection tray. The mixing chamber has anoutlet oriented to permit downward passage of liquid and vapor from themixing chamber. The distribution system further includes a spillwayextending through the collection tray to permit downward passage ofvapor and liquid from above the collection tray into the mixing chamber.The distribution system further includes a baffle connected to thecollection tray and extending downwardly therefrom into the mixingchamber. The baffle is located between the outlet and the spillway suchthat a baffle radius is greater than an outlet radius.

SUMMARY

The present application relates generally to hydrotreating processes andmore particularly, but not by way of limitation, to improved methods andsystems for reactant mixing and distribution in hydrotreating reactors.One aspect of the present invention relates to a mixing system for usein a chemical-process column. The mixing system includes aheavy-reactant mixing surface arranged perpendicular to a flow ofreactant through the chemical-process column. The mixing system alsoincludes an aperture formed in the heavy-reactant mixing surface. Apre-distributor is coupled to an underside of the mixing system andfluidly coupled to the aperture.

Another aspect of the present invention relates to a method of mixingreactants in a chemical-process column. The method includes introducinga heavy reactant to a mixing region of the chemical process column andcontacting a heavy reactant with a heavy-reactant mixing surface. Themethod further includes homogenizing the heavy reactant within a mixingpot. The method further includes distributing the heavy reactant via apre-distributor.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and system of the presentinvention may be obtained by reference to the following DetailedDescription when taken in conjunction with the accompanying drawingswherein:

FIG. 1 is a cross-sectional view of a reactor column according to anexemplary embodiment;

FIG. 2A is a top isometric view of a mixing system according to anexemplary embodiment;

FIG. 2B is an exploded perspective view of a heavy-reactant mixingsurface according to an exemplary embodiment;

FIGS. 3A-3H are detailed plan views of a central aperture according toexemplary embodiments;

FIG. 4 is a cross-sectional view of a mixing system taken across sectionline A-A of FIG. 2A according to an exemplary embodiment;

FIG. 5 is a top plan view of a mixing system according to an exemplaryembodiment;

FIG. 6 is a bottom isometric view of a mixing system according to anexemplary embodiment; and

FIG. 7 is a flow diagram of a process for mixing and distributing areactant according to an exemplary embodiment.

DETAILED DESCRIPTION

Various embodiments of the present invention will now be described morefully with reference to the accompanying drawings. The invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein; rather, the embodimentsare provided so that this disclosure will be thorough and complete, andwill fully convey the scope of the invention to those skilled in theart.

FIG. 1 is a cross-sectional view of a reactor column according to anexemplary embodiment. In a typical embodiment, a reactor column 100includes an upper catalyst bed 102 disposed above a lower catalyst bed104. A feed device 107 and a primary distributor 109 are disposed abovethe upper catalyst bed 102. A mixing region 106, including a mixingsystem 108 and a secondary distributor 110, is arranged between theupper catalyst bed 102 and the lower catalyst bed 104. In a typicalembodiment, the upper catalyst bed 102 and the lower catalyst bed 104are filled with a plurality of generally spherical catalyst pellets (notexplicitly shown). In various embodiments, particularly inhydrodesulfurization processes, the catalyst may include an alumina baseimpregnated with, for example, cobalt (Co) and molybdenum (Mo), known asa “CoMo catalyst.” In certain other embodiments, a catalyst containing acombination of nickel (Ni) and molybdenum (Mo), known as a “NiMocatalyst,” is utilized.

Still referring to FIG. 1, during operation, reactants are introduced tothe reactor column 100 via the feed device 107. For example, inhydrodesulfurization processes, the reactants may include a heavyreactant such as, for example, ethanethiol (C₂H₅SH) and a light reactantsuch as, for example, hydrogen vapor (H₂). The reactants descend throughthe primary distributor 109, through the upper catalyst bed 102, andreact on a surface of the catalyst pellets. The reactants enter themixing region 106 where the reactants pass through the mixing system 108and the secondary distributor 110. The mixing system 108 blends andhomogenizes the reactants thereby removing concentration gradients andtemperature gradients resulting from uneven or partial reaction of thereactants in the upper catalyst bed 102. The secondary distributor 110evenly distributes the reactants across a cross-sectional area of thereactor column 100. After leaving the mixing region 106, the reactantsmove to the lower catalyst bed 104 for further reaction.

FIG. 2A is a top isometric view of a mixing system according to anexemplary embodiment. In a typical embodiment, the mixing system 108includes a heavy-reactant mixing surface 202, at least onelight-reactant duct 204 is disposed on the heavy-reactant mixing surface202, and a pre-distributor 206 disposed below the heavy-reactant mixingsurface 202. In a typical embodiment, the mixing system 108 is orientedgenerally perpendicular to a flow of reactants within the reactor column100 (illustrated in FIG. 1). The mixing system 108 is generally circularwith a diameter that is generally coextensive with an inner diameter ofthe reactor column 100. The heavy-reactant mixing surface 202 includes aplurality of generally wedge shaped sections 208(1)-(8). A plurality ofbaffles 210(1)-(8) are arranged on the heavy-reactant mixing surface202. The plurality of baffles 210(1)-(8) are arranged generallyperpendicular to the heavy-reactant mixing surface 202.

FIG. 2B is an exploded view of the heavy-reactant mixing surface 202.The generally wedge shaped section 208(1) includes a body portion201(1), a first flange 203(1) formed along a first edge 211(1), and asecond flange 205(1) formed along a second edge 213(1). The first flange203(1) and the second flange 205(1) are formed generally perpendicularto the body portion 201(1). The generally wedge shaped sections208(2)-(8) are similar in terms of construction and operation to thegenerally wedge shaped section 208(1).

Referring to FIGS. 2A-2B, the plurality of generally wedge shapedsections 208(1)-(2) are assembled such that the first edge 211(1) of thegenerally wedge shaped section 208(1), abuts the second edge 213(2) ofthe generally wedge shaped section 208(2). The first flange 203(1) abutsthe second flange 205(2) to form the baffle 210(1). Similarly, theplurality of generally wedge shaped sections 208(2)-(3) are assembledsuch that the first edge 211(2) of the generally wedge shaped section208(2) is arranged to abut the second edge 213(3) of the generally wedgeshaped section 208(3). The first flange 203(2) abuts the second flange205(3) to form the baffle 210(2). The generally wedge shaped sections208(3)-(8) are assembled in similar fashion thereby forming the baffles210(3)-(8). The plurality of generally wedge shaped sections 208(1)-(8)are thus arranged into a generally annular shape.

Referring again to FIG. 2A, a central aperture 212, defined by theplurality of generally wedge shaped sections 208(1)-(8), is disposed inan approximate center of the heavy-reactant mixing surface 202. Duringoperation, heavy reactant contacts the heavy-reactant mixing surface202. The heavy reactant is directed towards the central aperture 212 bythe plurality of baffles 210(1)-(8). Mixing and homogenization of theheavy reactant occurs in the central aperture 212. In other embodiments,mixing systems utilizing principles of the invention may include adifferent number of generally wedge shaped sections. In still otherembodiments, mixing systems utilizing principles of the invention mayinclude a unitary mixing surface thereby omitting the generally wedgeshaped sections 208(1)-(8). In such embodiments, the baffles 210(1)-(8)may be coupled to the heavy-reactant mixing surface through a processsuch as, for example, welding, soldering, or the like.

FIGS. 3A-3H are detailed plan views of the central aperture 212according to exemplary embodiments. Referring first to FIG. 3A, a firstvane 301 and a second vane 321 are arranged around a perimeter of thecentral aperture 212 and disposed between adjacent ones of the pluralityof baffles 210(1)-(8). The first vane 301 and the second vane 321include a substantially right-angle section 302. The substantiallyright-angle section 302 disrupts flow of heavy reactant entering thecentral aperture 212 thereby inducing turbulent mixing of the heavyreactant.

Referring to FIG. 3B, a first vane 303 and a second vane 306 arearranged around the perimeter of the central aperture 212 and disposedbetween adjacent ones of the plurality of baffles 210(1)-(8). The firstvane 303 and the second vane 306 are curved towards each other thuscreating a plurality of nozzles 304. The first vane 303 and the secondvane 306, in combination with the plurality of nozzles 304, disrupt flowof heavy reactant entering the central aperture 212 thereby inducingturbulent mixing of the heavy reactant.

Referring to FIG. 3C, a first vane 313 and a second vane 319 arearranged around the perimeter of the central aperture 212 and disposedbetween adjacent ones of the plurality of baffles 210(1)-(8). The firstvane 313 and the second vane 319 are curved away from each other. Thefirst vane 313 and the second vane 319 disrupt flow of heavy reactantentering the central aperture 212 thereby inducing turbulent mixing ofthe heavy reactant.

Referring now to FIG. 3D, a static mixer 305 is around the perimeter ofthe central aperture 212. The static mixer 305 is disposed betweenadjacent ones of the plurality of baffles 210(1)-(8). The static mixer305 may include, for example, a plurality of crimped sheets of material.The static mixer 305 induces turbulent mixing of the heavy reactantentering the central aperture 212.

Referring to FIG. 3E, a plurality of vanes 307(1)-(8) are arrangedaround the perimeter of the central aperture 212. The vane 307(1) isdisposed between the baffles 210(1)-(2). The vanes 307(2)-(8) arearranged similarly. The vanes 307(1), 307(3), 307(5), 307(7) are curvedin a direction opposite that of the vanes 307(2), 307(4), 307(6),307(8). The plurality of vanes 307(1)-(8) induces turbulent mixing ofthe heavy reactant entering the central aperture 212.

Referring to FIG. 3F, a first vane 309 and a second vane 320 arearranged around the perimeter of the central aperture 212 and disposedbetween adjacent ones of the plurality of baffles 210(1)-(8). The firstvane 309 and the second vane 320 are arranged generally parallel to eachother and are curved in a similar direction thus creating a stirringeffect of the heavy reactant entering the central aperture 212.

Referring to FIG. 3G, a vane 315 is arranged around the perimeter of thecentral aperture 212. The vane 315 is disposed between adjacent ones ofthe plurality of baffles 210(1)-(8). The vane 315 is curved thuscreating a stirring effect of the heavy reactant entering the centralaperture 212.

Referring to FIG. 3H, a vane 317 is arranged around the perimeter of thecentral aperture 212. The vane is disposed between adjacent ones of theplurality of baffles 210(1)-(8). The vane 317 includes a right-anglesection 318. The right-angle section 318 disrupts flow of the heavyreactant thereby further inducing turbulent mixing of the heavy reactantentering the central aperture 212.

FIG. 4 is a cross-sectional view of the mixing system 108 taken acrosssection line A-A of FIG. 2A. The at least one light-reactant duct 204 isdisposed through the heavy-reactant mixing surface 202. The at least onelight-reactant duct 204 includes an aperture 402 having a cover 404disposed thereabove. The aperture 402 is fluidly coupled to a riser 406and is, thus, positioned above the heavy-reactant mixing surface 202.The riser 406 is fluidly coupled to a vectoring member 408 disposedbelow the heavy-reactant mixing surface 202. The vectoring member 408may include, for example, a nozzle, a tube, a vane, or any otherappropriate device as dictated by design requirements. The at least onelight-reactant duct 204 permits passage of the light reactant from aregion above the heavy-reactant mixing surface 202 to a region below theheavy-reactant mixing surface 202. The vectoring member 408 impartsvelocity and turbulence to the light reactant thereby improving mixingand homogenization thereof.

FIG. 5 is a top plan view of the mixing system 108. The baffles210(1)-(8) may be arranged at any angle with respect to a vertical axis527 and a horizontal axis 529. In the embodiment shown in FIG. 5, aplurality of light-reactant ducts 204 are arranged in a generallycircular fashion around the central aperture 212. In other embodiments,the at least one light-reactant duct 204 may be arranged in anyconfiguration as dictated by design requirements. The at least onelight-reactant duct 204 permits passage of the light reactant from aregion above the heavy-reactant mixing surface 202 to a region below theheavy-reactant mixing surface 202.

FIG. 6 is a bottom isometric view of the mixing system 108 according toan exemplary embodiment. The pre-distributor 206 is coupled to anunderside of the mixing system 108. The pre-distributor 206 includes atleast one channel 606 fluidly coupled to a mixing pot 602. The mixingpot 602 is in fluid communication with the heavy-reactant mixing surface202 via the central aperture 212 (shown in FIG. 2A). The mixing pot 602is disposed on a bottom surface of the mixing system 108 below theheavy-reactant mixing surface 202. In various embodiments, additionalvanes, fins, tabs, or other turbulence-inducing features may also bepresent within the mixing pot 602. A plurality of perforations 604 arepresent in a bottom surface 605 of the mixing pot 602. In a typicalembodiment, turbulent mixing and blending of the heavy reactant isinduced upon entering the mixing pot 602. Mixing and blending of theheavy reactant homogenizes the heavy reactant thereby removingconcentration and temperature gradients that may be present due topartial or uneven reaction.

Still referring to FIG. 6, the at least one channel 606 extendsoutwardly in a radial fashion from the mixing pot 602. The at least onechannel 606 includes a plurality of perforations 608 formed therein.During operation, the at least one channel 606 distributes homogenizedheavy reactant over a cross sectional area of the reactor column 100(shown in FIG. 1). In a typical embodiment, the at least one channel 606includes a generally square or rectangular profile. However, in otherembodiments, alternative profile shapes such as, for example, round,triangular, or polygonal could be utilized. In various embodiments, theat least one channel 606 includes flanged or welded pipes. In variousembodiments, the at least one channel 606 may include any number ofchannels. Furthermore, in various embodiments, the at least one channel606 may be arranged at any angle relative to each other or to thehorizontal axis 529 or the vertical axis 527.

FIG. 7 is a flow diagram of a process for mixing and distributingreactants according to an exemplary embodiment. The process 700 beginsat step 702. At step 704 reactants including, for example, a lightreactant and a heavy reactant, enter the reactor column 100, descendthrough the upper catalyst bed 102, and enter the mixing region 106. Atstep 706, the heavy reactant contacts the heavy-reactant mixing surface202 of the mixing system 108. At step 707, the light reactant enters theat least one light-reactant duct 204 and is directed beneath the mixingsystem 108. At step 708, the baffles 210(1)-(8) direct the heavyreactant towards the central aperture 212. At step 709, the lightreactant exits via the vectoring member 408 such as, for example,nozzles, tubes, vanes, and the like. At step 710, as shown in FIGS.3A-3H, in various embodiments, turbulent mixing and blending of theheavy reactant is induced upon entering the central aperture 212. In atypical embodiment, the mixing and blending homogenizes the heavyreactant thereby removing concentration and temperature gradients thatmay be present due to partial or uneven reaction.

Referring still to FIG. 7, at step 712, the heavy reactant enters themixing pot 602 where further blending of the heavy reactant occurs. Atstep 714, a portion of the heavy reactant exits the mixing pot 602 via,for example, the plurality of perforations 604 disposed on the bottomsurface 605 of the mixing pot 602. At step 716, remaining heavy reactantflows through all or a portion of the at least one channel 606. At step718, the remaining heavy reactant exits the at least one channel 606through the plurality of perforations 608. Thus, the at least onechannel 606 distributes homogenized heavy reactant over a crosssectional area of the reactor column 100. The process 700 ends at step720.

The advantages attendant to the mixing system 108 will be apparent tothose skilled in the art. First, the mixing system 108 combines thefunctions of reactant mixing and reactant pre-distribution into a singlecomponent thus allowing for cheaper and less complicated assembly andmaintenance. Moreover, the mixing system 108 allows for increasedcatalyst volume or a smaller column profile. In addition, as shown inFIGS. 3A-3H, the mixing system 108, induces turbulent mixing andblending of heavy reactant entering the central aperture 212. Suchturbulent mixing and blending improves over designs where reactants aresimply stirred or swirled. Stirring reactants often does not induceturbulent mixing and, as a result, often does not result in homogenizedreactants.

Although various embodiments of the method and system of the presentinvention have been illustrated in the accompanying Drawings anddescribed in the foregoing Detailed Description, it will be understoodthat the invention is not limited to the embodiments disclosed, but iscable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the invention as set forth herein.

What is claimed is:
 1. A method of mixing reactants in a chemical-process column, the method comprising: introducing a heavy reactant to a mixing region of the chemical process column; contacting the heavy reactant with a heavy-reactant mixing surface; homogenizing the heavy reactant within a mixing pot; and distributing the heavy reactant via a pre-distributor.
 2. The method of claim 1, comprising directing the heavy reactant toward the mixing pot via a baffle.
 3. The method of claim 1, comprising mixing the heavy reactant with a vane disposed at an entrance to the mixing pot.
 4. The method of claim 1, comprising mixing the heavy reactant via at least one of a vane, a deflector, a baffle, or a static mixer disposed within the mixing pot.
 5. The method of claim 1, comprising: introducing a light reactant to the mixing region of the chemical process column; directing the light reactant through the heavy-reactant mixing surface via a light-reactant duct; and mixing the light reactant via a vectoring member fluidly coupled to the light-reactant duct.
 6. The method of claim 5, wherein the vectoring member is arranged proximate to the pre-distributor.
 7. The method of claim 1, wherein the distributing comprises directing the heavy reactant through a plurality of channels fluidly coupled to the mixing pot, the plurality of channels having a plurality of perforations formed therein.
 8. The method of claim 1, wherein the homogenizing comprises inducing turbulent mixing of the heavy reactant within the mixing pot.
 9. The method of claim 1, wherein the homogenizing removes concentration and temperature gradients from the heavy reactant. 